1. Field of the Invention
The present invention relates to a load failure protection circuit of a discharge lamp driving apparatus, which turns on a discharge lamp for back lighting in a liquid crystal display panel, for example, and the discharge lamp driving apparatus. In particular, the present invention relates to drastic improvement of a load failure protection circuit which enables detection of a load failure, such as a short circuit between a high-voltage side and a low-voltage side of each discharge lamp, in a discharge lamp driving apparatus which performs lighting drive of a plurality of discharge lamps.
2. Description of the Prior Art
Conventionally, for example, what make one or more cold cathode lamps (hereafter, these are called CCFL(s)) discharged and lighted simultaneously for back lighting of various kinds of liquid crystal display panels used for a liquid crystal television set and the like are known. Various apparatuses as a discharge lamp driving apparatus for that are proposed (e.g., refer to Japanese Unexamined Patent Publication No. 2004-265868).
The discharge lamp driving apparatus disclosed by the above-mentioned patent publication not only detects a current which flows through each CCFL, but also gives feedback of a voltage, equivalent to the full current value, to a control circuit. Then, this has the structure that drive of CCFLs is performed so that a sum total of currents which flow through respective CCFLs may become always fixed on the basis of this feedback voltage.
By the way, a CCFL not only needs a high voltage of about 800 V between both ends at the time of usual lighting, but also needs a high voltages which is about 2 to 2.5 times the voltage at the time of usual lighting, at the time of a lighting start. Therefore, when a damage of a CCFL, a short circuit in its connecting terminals, and the like arise, there is a possibility of causing: dielectric breakdown by rise of a secondary side voltage caused by an open state at the secondary side of a transformer used for a drive circuit; burning of the transformer by a short-circuit current being generated; and the like. Hence, it is important to consider enough circuit protection to such a load failure condition.
Nevertheless, the apparatus disclosed in the above-mentioned patent publication maintains a full current value always uniform even if some dispersion in drive currents arises in each CCFL at the time of normal operation, and can prevent a damage and a malfunctioning by an extreme current flowing into one CCFL. Hence, the apparatus is not constructed to fully respond to the above load failure conditions.
What has been already proposed as a circuit which detects a load failure condition of a discharge lamp driving apparatus is, for example, a circuit having structure shown in FIG. 4.
Hereafter, the circuit (hereafter, this is called a conventional circuit) which relates to this conventional technology is outlined with referring to this diagram.
First, main components of this conventional circuit are two N channel MOSFET transistors (hereafter, these are called FETs) 2 and 3 which construct a push-pull circuit, two transformers 4 and 5, and a control IC 1 which is made an integrated circuit, and makes up an inverter-controlled drive circuit for discharge and lighting of two CCFLs 201 and 202. Furthermore, this conventional circuit is constructed so as to detect a failure between terminals to which the CCFLs 201 and 202 are connected, and to make the control IC 1 stop drive of the CCFLs 201 and 202 when a failure is detected. Hence, a tube current detector 105 which detects a failure condition is provided.
The FETs 2 and 3 which form a push pull circuit are constructed so as to be switching if driven by the control IC 1 to be able to apply AC voltages to primary sides of the transformers 4 and 5 which correspond respectively. Then, high voltages are generated in secondary sides of transformers 4 and 5, which are applied to a first CCFL 201 connected between first and second terminals 6a and 6b, and a second CCFL 202 connected between third and fourth terminals 7a and 7b respectively.
The tube current detector 105 is constructed of two NPN transistors 61 and 62 as main components, and operates so as to detect presence of an failure at the first and second terminals 6a and 6b, and the third and fourth terminals 7a and 7b to which the CCFLs 201 and 202 are connected respectively to make the control IC 1 stop drive of the CCFLs 201 and 202 when detecting an failure.
Specifically, first, in a state that the two CCFLs 201 and 202 are operating normally, voltages according to currents which flow through the CCFLs 201 and 202 respectively are obtained by rectification circuits 23a and 23b, and resistors 63a and 63b which are connected to these rectification circuits 23a and 23b in parallel. Then, a voltage according to a current which flows through the first CCFL 201 connected between the first and second terminals 6a and 6b is applied to a base of a transistor 61 through a resistor 64. In addition, a resistor 66 is connected between a base and an emitter of the transistor 61. On the other hand, a voltage according to a current which flows through the second CCFL 202 connected between the third and fourth terminals 7a and 7b is applied to a feedback terminal F/B of the control IC 1. Nevertheless, a collector of the transistor 62 is connected to this voltage line.
Hence, at the time of CCFL normal operation, the transistor 61 is made a conductive state by the voltage applied to its base, and the base of the transistor 62 is made almost ground potential. Hence, the transistor 62 becomes a nonconductive state. Therefore, a predetermined rectification voltage obtained by the rectification circuit 23b is applied to the feedback terminal F/B of the control IC 1 as it is.
Then, in a state that the predetermined rectification voltage is applied to the feedback terminal F/B, the control IC 1 consecutively drives the CCFLs 201 and 202 as operation of the CCFLs 201 and 202 being normal.
On the other hand, in the case of a load failure, for example, the first terminal 6a being grounded by a certain cause, a current does not flow between the first and second terminals 6a and 6b. Hence, a base voltage of the transistor 61 becomes almost 0 V, and the transistor 61 becomes a nonconductive state. Thereby, a predetermined voltage Vdd which is also a supply voltage of the control IC 1 is applied to the base of the transistor 62 with being divided by resistors 65a, 65b, and 65c. Hence, the transistor 62 becomes a conductive state, and ground potential is applied to the feedback terminal F/B of the control IC 1.
As a result, the control IC 1 is judged to be a load failure and stop the drive of the CCFLs 201 and 202.
As load failures, besides the above-mentioned short circuit of a high-voltage side terminal to a ground side, there is, for example, a case that the CCFL 201 or 202 is damaged by a certain cause so that the first terminal 6a and second terminal 6b become an open state, or the third terminal 7a and fourth terminal 7b become an open state.
In this case, a function of detecting such an open state and stopping the drive of the CCFLs 201 and 202 is added to the control IC 1. Specifically, secondary side voltages of the transformers 4 and 5 which are given capacitor division are applied to an open state voltage detection terminal OVP of the control IC 1 through a first diode 9a for open state voltage detection, or a second diode 9b for open state voltage detection. Then, when this applied voltage is judged to be beyond a predetermined value, the drive of the CCFLs 201 and 202 is stopped as inter-terminal open.
By the way, as load failures, besides the above-mentioned, a short circuit between a high voltage terminal and a low voltage terminal may be cited. For example, this is a case that anything intervenes by a certain cause between the first terminal 6a and second terminal 6b, and thereby, an overcurrent flows between terminals. Since such an overcurrent causes not only fuming and firing of the transformers 4 and 5, but also fuming and firing of other electronic parts, it is necessary to detect it surely, and to cut off the current.
Nevertheless, the above-described tube current detector 105 cannot detect surely this short circuit between a high voltage terminal and a low voltage terminal. That is, since a short-circuit current flows when the short circuit between the high voltage terminal and low voltage terminal occurs, a voltage which is sufficient for making the transistor 61 into a conductive state is applied to the base of the transistor 61. Hence, the transistor 61 becomes a conductive state, and on the other hand, the transistor 62 becomes a nonconductive state. Therefore, since the predetermined rectification voltage obtained by the rectification circuit 23b similarly at the time of normal operation is applied to the feedback terminal F/B of the control IC 1, the control IC 1 consecutively drive the CCFLs 201 and 202 in spite of the load failure.
In this way, a conventional circuit has structure of representing a voltage signal, obtained from one CCFL in the plurality of CCFLs 201 and 202, as a feedback signal offered in order to stop the drive of the CCFLs 201 and 202 in the case of a load failure arising, and deciding only presence of a tube current for the remaining CCFL. Hence, there was a problem that it was not possible to decide to be a load failure in a control circuit even if a short circuit between the high voltage side and low voltage side arose in the CCFL which did not generate the feedback signal, and it was not possible to perform preventive measures against the fuming, firing, and the like of a transformer, and the like resulting from the load failure and the like surely.